Most GTA titles on PC utilize a file named dinput8.dll (DirectInput 8) for controller inputs. The Ultimate ASI Loader exploits the Windows DLL Search Order. By placing a renamed loader file as dinput8.dll in the game root directory, the operating system loads the modder's code instead of the legitimate DirectInput library.
For organizational sanity, create a folder named asi inside the root directory. This is where you will place all your .asi mods. The Ultimate ASI Loader scans both the root folder and the asi folder, but keeping them separate prevents clutter. gta vice city ultimate asi loader
Since the loader itself is just a bridge, its "features" are actually the possibilities it unlocks: Most GTA titles on PC utilize a file named dinput8